The present invention relates to cordierite bodies of high permeability and tailored microstructure suitable for use as diesel particulate filters in applications where a low pressure drop across the length of the filter is required.
Recently much interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have come under attack both in the United States and Europe, for their harmful effects on the environment and on humans. As such, stricter environmental regulations will require diesel engines to be held to the same standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is faster, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
One of the biggest challenges in lowering diesel emissions is controlling the levels of diesel particulate material present in the diesel exhaust stream. In 1998 diesel particulates were declared a toxic air contaminant by the California Air Resources Board. Legislation has been passed that regulates the concentration and particle size of diesel particulate pollution originating from both mobile and stationary sources.
Diesel particulate material is mainly carbon soot. One way of removing the carbon soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the diesel particulate filter which filters the diesel exhaust by capturing the soot on the porous walls of the filter body. The diesel particulate filter is designed to provide for nearly complete filtration of soot without significantly hindering the exhaust flow.
In the industry cordierite (2MgO-2Al2O3-5SiO2) has been the cost-effective material of choice for diesel particulate filters for heavy duty vehicles due to its combination of excellent thermal shock resistance, filtration efficiency, and durability under most operating conditions. Historically, cordierite diesel particulate filters have had cell geometries such as 100 cell/in2 with 0.017 inch walls and 200 cell/in2 with 0.012 inch walls, with alternate channels plugged on opposite faces to force the engine exhaust gas to pass through the porous walls of the filter body.
As the layer of soot collects on the surfaces of the inlet channels of the diesel particulate filter, the lower permeability of the soot layer causes a gradual rise in the back pressure of the filter against the engine, causing the engine to work harder. Once the carbon in the filter has accumulated to some level, the filter must be regenerated by burning the soot, thereby restoring the back pressure to low levels. Normally, the regeneration is accomplished under controlled conditions of engine management whereby a slow burn is initiated and lasts a number of minutes, during which the temperature in the filter rises from about 400-600xc2x0 C. to a maximum of about 800-1000xc2x0 C.
The highest temperatures during regeneration tend to occur near the exit end of the filter due to the cumulative effects of the wave of soot combustion that progresses from the entrance face to the exit face of the filter as the exhaust flow carries the combustion heat down the filter. Under certain unusual circumstances, a so-called xe2x80x9cuncontrolled regenerationxe2x80x9d can occur when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas (such as engine idling conditions). During an uncontrolled regeneration, the combustion of the soot (a reaction which is already highly exothermic) may produce temperature spikes which would exceed the melting point of cordierite, and can thermally shock and crack, or even melt, the filter.
In addition to capturing the carbon soot, the filter also traps metal oxide xe2x80x9cashxe2x80x9d particles that are carried by the exhaust gas. These particles are not combustible and, therefore, are not removed during regeneration. However, if temperatures during uncontrolled regenerations are sufficiently high, the ash may eventually sinter to the filter or even react with the filter resulting in partial melting.
A significant problem associated with conventional cordierite diesel particulate filters is susceptibility to damage during regeneration of the filter under uncontrolled conditions that promote unusually high temperatures.
It would be considered an advancement in the art to obtain a cordierite diesel particulate filter which not only survives the numerous controlled regenerations over its lifetime, but also the much less frequent but more severe uncontrolled regenerations. This survival includes not only that the diesel particulate filter remains intact and continues to filter, but that the back pressure against the engine remains low.
The present invention provides such a filter and a method of making the same.
The instant invention is founded upon the discovery of a cordierite structure possessing high permeability and a microstructure with a unique combination of porosity and pore size distribution which is especially useful in the fabrication of diesel particulate filters which possess high thermal durability coupled with a low pressure drop along the length of the filter such that there exists a low back pressure against the engine, resulting in a more efficient engine.
The inventive structure comprises predominately a cordierite-type phase approximating the stoichiometry Mg2Al4Si5O18 and having a coefficient of thermal expansion (25-800xc2x0 C.) of greater than 4xc3x9710xe2x88x927/xc2x0 C. and less than 13xc3x9710xe2x88x927/xc2x0 C. and a permeability and a pore size distribution which satisfy the relation 2.108 (permeability)+18.511 (total pore volume)+0.1863 (percentage of total pore volume comprised of pores between 4 and 40 micrometers) greater than 24.6.
The inventive structure is suitable in high temperature applications such as a diesel particulate filter of high volumetric heat capacity and which exhibits a low pressure drop across the length of the filter. In a preferred embodiment the filter is a honeycomb design having an inlet end and an outlet end and a multiplicity of cells extending from the inlet end to the outlet end, the cells having porous walls, wherein part of the total number of cells at the inlet end are plugged along a portion of their lengths, and the remaining part of cells that are open at the inlet end are plugged at the outlet end along a portion of their lengths, so that an engine exhaust stream passing through the cells of the honeycomb from the inlet end to the outlet end flows into the open cells, through the cell walls, and out of the structure through the open cells at the outlet end.
The inventive filter has a pressure drop across the length of the filter, expressed in kPa at an artificial carbon soot loading of 5 grams/liter and a flow rate of 26 standard cubic feet per minute (scfm), of less than 8.9xe2x88x920.035 (number of cells per square inch)+300 (cell wall thickness in inches); a bulk filter density of at least 0.60 g/cm3 and a volumetric heat capacity of at least 0.67 J cm3 Kxe2x88x921 as measured at 500xc2x0 C.
The invention is also a method of making the cordierite body based upon the use of certain raw materials having specific particle size restrictions. Specifically the method includes forming a mixture of a talc source having a morphology index greater than about 0.75 and an average particle size greater than 15 micrometers but less than 35 micrometers; an alumina source having an median particle size between 4.6 and 25 micrometers; a silica source having a median particle size between 10 and 35 micrometers; shaping the mixture into a green structure; optionally drying and firing into a final product structure. Kaolin may be added but not be more than the quantity (in weight percentage) given by the equation 4.0 (median particle size of the alumina source)xe2x88x9218.4.